Since widespread use of integrated circuits began in the 1960s, the demand for data communications equipment has increased rapidly throughout the world. As more and more businesses, and even homes, come to rely on low-cost computers, there is a growing need for data communications devices, commonly known as modems, which may be used over dial-up voice grade telephone lines.
Due to the limited bandwidth of voice grade telephone lines, various schemes for modulating data in a form which can be transmitted over voice grade telephone lines have developed. In particular, several methods of encoding information by the relative phase of a signal transmitted over the telephone line have been developed to increase the rate at which data may be transmitted over a band limited channel of a specified bandwidth. One of the most popular forms of phase encoding has been differential four-phase, phase shift keyed (PSK) encoding. In this arrangement, a carrier of a constant frequency is transmitted and information is encoded by differences in the relative phase of the carrier which occur at predetermined "bit times". The phase distortion of the transmission channel is ultimately an upper limit. However, since the baud, or keying, rate for the transmission is directly proportional to the bandwidth of the channel, the approach has been to encode multiple bits of binary data for each keying of the transmitter. Thus, the more bits which can be encoded in each change of the output signal from the transmitter, the more information can be transmitted per unit time over a transmission channel of a specified bandwidth. Experience has taught that information can be transmitted reliably via this form of encoding over voice grade telephone lines and that transmitters and receivers for this type of signal may be economically made.
In differential four-phase PSK, a carrier signal of a specified frequency (usually either 1200 or 2400 Hz) is transmitted. Each keying of the transmitter (each "bit time") will cause the transmitted signal to be characterized by one of four possible phases relative to its previous phase. Since there are four possible changes for each keying of the transmitter, two bits may be encoded in each keying of the transmitter, and thus the scheme will transmit two bits per baud. A further specification of this popular form of data transmission is that the transmitter is operated at a 600 baud rate and thus 1200 bits per second may be transmitted.
As cheaper digital electronic circuits became available, a number of arrangements for modulating differential four-phase PSK have been incorporated. For example, one well known scheme used in Bell 212A modems has involved stuffing of bits into a bit stream in a shift register to effect the phase shifts. This arrangement is shown in U.S. Pat. No. 4,049,909 to Peck.
While it has long been recognized that digital arrangements for generating the PSK signal can be implemented more cheaply than most analog arrangements, many digital modulators have high levels of out-of-band signals and require more complex, and thus more expensive, band pass filters to be inserted between the output of the modulator and the telephone line carrying the signal.
It is known to those skilled in the art that it is highly desirable to use microprocessors in implementing modems. Microprocessors have been used in the past, to implement a number of different functions within a modem including demodulation of data. See for example U.S. Pat. No. 4,431,867 to Dale A. Heatherington which is assigned to the assignee of the present invention.
Thus, while digital modulators for differential PSK data transmission have been used, a significant amount of the cost savings has been offset by the requirement of increased filtering of the output signal. There is thus a need in the art to provide a digital differential PSK modulator, particularly one which may be implemented with apparatus which includes a microprocessor or one chip microcomputer, which produces lower levels of out-of-band signals, and thus requires less expensive transmit filters to be used.
The design of receivers is based on the premise that it is the relative phase between a present bit time and the previous bit time that is important to the receiver, and not the particular path by which a vector in a phase plane representing the relative phase of the carrier signal took to arrive at its new phase value. However, as more and more receivers for differential PSK employ phase lock loops as part of the receiving apparatus, the d.phi./dt characteristic of the transmission signal becomes more important. For example, for transmission at commonly used 600 baud/1200 bit per second four-phase PSK, a succession of 90.degree. phase shifts will cause the phase lock loop in the receiver to detect a shift in the carrier frequency in excess of 100 Hz making tracking difficult.
Thus, there is a need in the art to further provide digital modulators which not only reduce the out-of-band transmission which must be filtered by relatively expensive analog transmit filters, but also present a signal which is relatively easy for a phase lock loop in a receiver to lock onto.